Ultra-short pulsed fiber laser systems (“fiber lasers”) present an attractive alternative to their solid-state analogues. One obvious advantage is size; an optical fiber can be easily coiled with a small diameter. Another advantage is efficiency. Fiber lasers based on Yb-doped fiber operating in the 1030-1100 nm spectral range can be very efficiently pumped with inexpensive, reliable and high-brightness 980 nm laser diodes. Yet another advantage is the beam quality at high power that results because the optical fibers in the system can be designed to support only the fundamental waveguide mode. This provides a nearly Gaussian output beam for average powers up to 100 W and above, with several kW of output power demonstrated in single-mode continuous-wave (CW) fiber lasers. Another important advantage of fiber lasers is their greatly simplified assembly and maintenance. Parts of the system can be optically coupled by fusion splicing, which obviates the need for fiber re-alignment.
A major obstacle that limits the amount of power in a high-output pulsed fiber laser is nonlinear distortion of the optical pulses. While non-linear effects can be reduced by increasing the fiber core size, the core size cannot be increased indefinitely without making the fiber multimode, which would eliminate the high beam quality advantage associated with single-mode operation, and would cause the unrecoverable broadening of the laser pulses due to the mode dispersion in multimode fiber. Amplifying the light pulses to a high energy (and therefore a high peak power) produces significant spectral broadening due to a nonlinear effect known as “self-phase modulation” that, when coupled with chromatic dispersion, causes an unrecoverable change of the pulse shape.
To avoid such non-linearities, in practical fiber lasers the pulses are usually “stretched” by passing them through a highly dispersive element (e.g., a delay line) prior to amplification, so that the peak power and therefore the amount of self-phase modulation is greatly reduced. Amplification of light pulses to an energy exceeding 1 mJ in a fiber amplifier with subsequent high-quality compression back to ˜400 fs duration has been demonstrated and shown to require stretching to >1 ns even for the fiber amplifier with the fiber core diameter as large as 50 μm. The pulse stretching is commonly accomplished using non-fiber (i.e., external) bulk diffraction grating, which requires that the light be taken out of the fiber, passed through a number of bulk optic elements (requiring precise alignment), and then coupled back in the fiber for amplification. While effective, the use of such external components negates several important advantages of fiber lasers—namely, their integrated (i.e., all-fiber) construction, their lack of need for precise alignment, and their compactness.
A number of attempts have been made to use a dispersive delay line consisting of a segment of optical fiber spliced between a fiber oscillator and a fiber amplifier, thus keeping a system all-fiber at least up to a final compression stage. Unfortunately, there are no fiber designs suitable for pulse compression when the pulse energy exceeds a few μJ.
The chromatic dispersion of a standard silica-based step-index single-mode fiber in the 1000 nm wavelength range is about −40 ps/nm/km. This is sufficient to stretch 100 femtosecond (fs) pulses to about 0.5 ns duration in 1 km of fiber length. Unfortunately, not any optical fiber can be used to stretch the pulse because the properties of the fiber do not allow for the stretched pulse to be converted back to the short (e.g., femtosecond) duration using bulk diffraction grating-based compressors. This is because the typical single-mode fiber has a relatively large third order dispersion (a.k.a. dispersion slope) and fourth order dispersion (a.k.a. dispersion curvature). The known bulk diffraction grating compressor designs usually have positive third order dispersion and therefore cannot compensate for the (also positive) third order dispersion of the fiber used to stretch the pulse. The result is a distorted pulse whose power is spread out into the “wings” surrounding the main power peak. An additional third order dispersion compensator using bulk glass prisms can be built to try to obtain a good pulse shape, but such a design increases the size and complexity of the system as a whole and creates additional loss, which translates into less output power.